In electrostatographic imaging and recording processes, for instance, in electrophotographic copying processes, an electrostatic latent image formed on a photoconductive surface is developed with a thermoplastic toner powder which is thereafter fused to a substrate. The fusion step commonly consists of passing the substrate, such as a sheet of paper on which toner powder is distributed in an image pattern, through the nip of a pair of rolls. At least one of the rolls is heated and has a resilient surface. Resilience is desirable so that the resilient roll will deform somewhat under pressure and create a significant area of contact with the other roll at the nip. In this way the residence time of toner in the heated nip is sufficient to fuse it to the paper.
A persistent problem with toner fusing devices is that when the toner is heated during passage through the rolls it may adhere not only to the paper but also to the fusing roll which contacts it. Any toner remaining on the roll can cause a false offset image to appear on the next sheet that passes through the nip and can also degrade the performance of the fusing roll.
To prevent toner offset many expedients have been tried such as covering the rolls with fluorocarbon polymers or silicone polymers of low surface energy. Also, poly(dimethylsiloxane) (also referred to as PDMS) oils have been applied as release liquids to the roll surfaces. With such materials, however, problems can occur.
One problem is that fluorocarbon polymers are difficult to wet with PDMS release oils, and the application of amounts sufficient to wet the roll can cause oil stains on the paper to which toner is being fused.
Although PDMS oils can prevent the build-up of toner on fusing rolls, they cause another problem, because they are compatible with poly(dimethylsiloxane) rubbers that are widely used as fusing roll covers. The poly(dimethylsiloxane) oils are absorbed by the poly(dimethylsiloxane) covering of the rolls upon repeated use and cause swelling of the rolls.
Because of the swelling of the rolls, defects appear in thermally fixed images. In particular, "step patterns" appear in the images when using various copy sheet sizes. These result from the differential swelling of the fuser roll inside and outside of the paper path, which causes non-uniform roll compression when different sizes of copy paper are used. There can also be increased wear on the roll and shortened useful fusing roll life, because of softening of the roll surface and degrading interaction of PDMS oil with the core or with adhesive interlayers.
Another fault of poly(dimethylsiloxane) rubber polymers as fuser roll covers is that they have a lower than desirable thermal conductivity, which leads to inefficient heating of the toner. Also, if the fuser roll is internally heated, low thermal conductivity requires high temperatures that can shorten the life of the fuser roll by causing thermal degradation, especially at the interface of the core and cover of the roll.
U.S. Pat. No. 4,430,406, discloses that swelling of a fusing roll can be controlled by spraying a fluorocarbon elastomer overcoat on the silicone elastomer roll cover. This method is costly, however, and only partially solves the problem.
U.S. Pat. No. 4,515,884 discloses the use of a release oil having a viscosity in the range from 7,000 to 20,000 centistokes, which allegedly reduces the problem. Even these materials, however, can lead to step patterns in the images.
Because of the swelling problems encountered with poly(dimethylsiloxane), it has been suggested in Japanese Kokai No. 59-209129, published Nov. 27, 1984, to make the roll covering from polymers containing some methylphenylsiloxane repeating units. Such polymers would perhaps be less compatible with poly(dimethylsiloxane) oils and hence less subject to swelling. Unfortunately, such polymers can degrade during preparation or use to yield siloxane compounds having single phenyl groups attached to silicon, which have been shown to exhibit estrogenic and sterility effects, making them undesirable for human contact.
A need exists for a fusing device having a resilient layer that has the useful properties of poly(dimethylsiloxane) elastomers but that is more thermally conductive, is more resistant to swelling by PDMS release oils, is thermally stable, and that does not yield estrogenic degradation products. The present invention provides such a fusing device and a method for using it.